This invention relates to a method of exposing a photosensitive material and, more particularly, to a method of exposing a photosensitive material of the direct lithographic printing plate making type to an original composed of a continuous tone image mingled with a line image by use of a process camera of the daylight type which permits the exposure and associated operation to be carried out in daylight.
At present, a variety of sensitive materials convertible to lithographic printing plates are known and are in actual use. As typical examples, there may be mentioned presentized plates (hereinafter referred to briefly as PS plates) employing diazonium compounds, electrophotographic plates employing zinc oxide or organic conductors, and silver salt photographic plates employing silver halide emulsions. Especially the photosensitive lithographic materials of high sensitivity such as the second and third ones of the above sensitive materials have come into remarkably wide use in the direct making of lithographic printing plates, because such sensitive materials can be treated conveniently and rapidly in an automatic printing plate processor throughout the process beginning with photographing of the original to be printed and including a sequence of processing steps.
Being very high in sensitivity, contrast, and resolving power, the silver salt sensitized material is especially useful. However, although having been imparted with excellent characteristics and improved quality, such sensitized materials are still unsuitable at present for use in the direct making of lithographic printing plates capable of reproducing multicolor prints from a continuous gradation color original such as multicolor photographs. The reasons for this may be many, but one of the most important is a considerably inferior image reproducibility together with an insufficient printing endurance of the directly prepared printing plate as compared with a PS plate. As known well, the PS plate is imparted with a high image-reproducibility through contact printing from an expensive lith film used as an intermediate original, whereas the direct plate making process involves photographing in a camera through a lens, which results an insatisfactory image reproducibility. As described in Japanese Patent Application "Kokai" (Laid-open) No. 89,007/73, the process camera includes a prism or a reflection mirror to obtain a laterally corrected image. Such a prism or mirror is known to exert an influence upon optical characteristics, resulting in deterioration of the image reproducibility. Every process camera now in general use photographs a reflected image. Accordingly, the originals to be photographed are so-called reflection images such as characters and lines (these images are referred to as line images) formed on high-contrast image-forming materials employing an opaque support, such as photographic composing paper sheets or positive paper sheets used in the silver complex diffusion transfer process; or other reflection images such as a halftone dot image transformed from a continuous gradation image, e.g., a photograph. A general practice is to arrange the reflection images according to a predetermined design and use as the finished original (block copy).
When a lithographic printing plate is directly made from a silver salt sensitized plate or electrophotographic plate with a reflection halftone dot image used as orginal, there is reproduced a practically acceptable halftone dot image having a screen ruling of generally 80 or at most 100 lines per inch. Accordingly, even if black and white halftone dot images are prepared by using a panchromatically sensitized continuous gradation film or lith film for color separation and if lithographic printing plates for color printing are prepared by the direct process from said halftone dot images used as original, the resulting printing plates produce a printed color image with unsatisfactory image reproducibility.
In order to solve the above problems, U.S. application Ser. No. 697,569 has proposed a method in which a sensitive material of the lithographic type having a spectrally sensitized panchromatic emulsion layer is brought into direct close contact with a contact screen and exposed through color separation filters. This method comprises placing the sensitive material upon a contact screen disposed on a horizontal transparent platen, then imagewise exposing the underside of the resulting assembly, and manually removing the contact screen from the camera. The sensitive surface tends to get scratched in inserting and removing the contact screen, resulting in scatch fogging and scumming. Such a defect is exaggerated by the presence of a fine powder of several .mu.m in particle size (e.g. 2 to 10 .mu.m) which is used to improve the water retention of the constituent layers of sensitive materials for the direct making of lithographic printing plates. In the direct making of multicolor printing plates, a plurality of printing plates such as cyan, magenta, yellow, and black plates must be precisely made, because these plates are used in superimposed impression to obtain a multicolor print; otherwise a print with satisfactory color reproducibility is not obtained from an original of a continuous tone image (e.g. color photograph) mingled with a line image (e.g. letters). In some of the commercially available process cameras of the daylight type such as those described in Japanese Patent Application "Kokai" (Laid-open) Nos. 70,542/81 and 204,032/84, a contact screen fixed in a frame is manipulated to approach a sensitive material fixed to an exposure platen, which is provided with a suction mechanism and disposed in nearly vertical positin, until a close contact is established between them to make them ready for the imagewise exposure. However, since such a process camera is designed to photograph a line image and a continuous tone image on two separate sensitive materials, it is impossible to form in daylight a line image and dot image on the same sensitive material.